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GENERAL DESCRIPTION OF MAJOR DIVISIONS AND CLASSES

PROKARYOTESCYANOPHYTA (BLUE-GREEN ALGAE)

• Historically, the major groups of algae are classified into Divisions (the equivalent taxon in the zoological code was the Phylum) on the basis of – pigmentation, – chemical nature of photosynthetic storage product, – photosynthetic membranes’ (thylakoids) organization and other features

of the chloroplasts, – chemistry and structure of cell wall, – number, arrangement, and ultrastructure of flagella (if any), – occurrence of any other special features, and sexual cycles.

• Recently, all the studies that compare the sequence of macromolecules genes and the 5S, 18S, and 28S ribosomal RNA sequences tend to assess the internal genetic coherence of the major divisions. This confirms that these divisions are non-artificial, even though they were originally defined on the basis of morphology alone.

Table 1.4 attempts to summarize the main characteristics of the different algal divisions.

Predominant photosynthetic pigments, storage products and cell wall components for the major algal groups.

• The cyanophytes, or Cyanoabacteria, comprise a single class of Cyanophyceae or blue-green algae

• They are, today, usually referred to as the cyanobacteria (blue green bacteria).

• The term cyanobacteria acknowledges that these prokaryotic algae are more closely related to the prokaryotic bacteria than to eukaryotic algae.

• Cyanobacteria have chlorophyll a (some also have chlorophyll b or d), phycobiliproteins, glycogen as a storage product, and cell walls containing amino sugars and amino acids.

• At one time, the occurrence of chlorophyll b in cyanobacteria was used as a criterion to place the organisms in a separate group, the Prochlorophyta. Modern nucleic-acid sequencing, however, has shown that chlorophyll b evolved a number of times within the cyanobacteria and the term Prochlorophyta has been discarded

PROKARYOTESCYANOPHYTA (BLUE-GREEN ALGAE)• cyanobacteria are possibly the most widely distributed

of any group of algae and inhabit marine and freshwater environments, moist soils and rocks, either as free-living or as symbiotic organisms.

• They are – planktonic, occasionally forming blooms in eutrophic lakes, and

are an important component of the picoplankton in both marine and freshwater systems;

– benthic, as dense mats on soil or in mudflats and hot springs, as the “black zone” just above the mean high water level on the seashore, and as relatively inconspicuous components in most soils; and

– symbiotic in diatoms, ferns, lichens, cycads, sponges, and other systems.

• Numerically these organisms dominate the ocean ecosystems.– There are approximately 1024 cyanobacterial cells in the oceans.

To put that in perspective, the number of cyanobacterial cells in the oceans is two orders of magnitude more than all the stars inthe sky.

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Morphology• Unicells, free-living or enclosed within a mucilaginous envelope. • A row of cells called a trichome. When the trichome is surrounded

by a sheath, it is called a filament.

Trichome of Arthrospira sp. (Bar: 20 µm.) Cells of Prochloron sp. (Bar: 10 µm.)

Morphology• It is possible to have more than one trichome in a filament.

• The most complex thallus is the branched filament. Such a branched filament can be uniseriate (composed of a single row of cells) or multiseriate (composed of one or more rows of cells).

Cell wall and gliding

• The cell wall of cyanobacteria is basically the same as the cell wall of Gram-negative bacteria. A peptidoglycan layer is outside of the cell membrane.

Transmission electron micrographs of sections of the wall of the cyanobacterium Phormidium uncinatum. The cell wall (CW) contains layers similar to those of a Gram-negative bacterium, e.g., the cytoplasmicmembrane (CM), peptidoglycan layers (P), periplasmic space (PS) and outer membrane (OM). In addition, the cyanobacterium contains the additional two external layers typical of a motile cell, the serrated externallayer (EL) and hair-like fibers (F). (CJ) Circumferential junction; (JP) junctional pore.

• The peptidoglycan is an enormous polymer composed of two sugar derivatives, N-acetylglucosamine and N-acetylmuramic acid, and several different amino acids

• Outside of the peptidoglycan is a periplasmic space (PS), probably filled with a loose network of peptidoglycan fibrils. An outer membrane (OM)surrounds the periplasmic space.

Fig. 2.2 The structure of a peptidoglycanmolecule in the cell wall of cyanobacteria.

Transmission electron micrographs of sections of the wall of the cyanobacteriumPhormidium uncinatum. The cell wall(CW) contains layers similar to those of a Gram-negative bacterium, e.g., the cytoplasmic membrane (CM), peptidoglycan layers (P), periplasmicspace (PS) and outer membrane (OM).

Gliding• Some cyanobacteria are capable of gliding, that is, the

active movement of an organism on a solid substrate where there is neither a visible organ responsible for the movement nor a distinct change in the shape of the organism.

• Gliding is a slow uniform motion at a direction parallel to the long axis of the cell and is occasionally interrupted by reversals in direction.

• Gliding is accompanied by a steady secretion of slime, which is left behind as a mucilaginous trail.

• Some cyanobacteria rotate during gliding while other cyanobacteria do not rotate.

• The cell wall of gliding bacteria has two additional layers outside of the cell wall.

• A serrated external layer (S-layer) and a layer of hair-like fibers occur outside of the outer membrane of the cell wall of gliding cyanobacteria.

• The hair-like fibers of the outermost layer are composed of a rod-like glycoprotein called oscillin.

Cross section wall of gliding bacteriaCross section wall of not capable of gliding bacteria

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• The cross walls of neighboring cells of gliding cyanobacteria contain junctional pores that are 15 nm in diameter and radiate outward from the cytoplasm at an angle of about 30–40° relative to the plane of each septum.

• The number of rows of junctional pores around each side of the septum varies from one circumferential ring to several rows of pores that girdle the septum.

• The junctional pore is 70–80 nm long and spans the entire multilayered cell wall.

• The junctional pore is composed of a tube-like base and an outer pore complex.

• Gliding occurs by slime secretion through the circumferential junctionalpores on one side of the septum.

• The slime passes along the surface of the oscillin fibers of the outer layer of the cell wall and onto the adjacent substrate, propelling the filament forward.

• The orientation of the oscillin fibers of the outer layer determines whether the filament rotates during gliding. – In Anabaena, the spiral oscillin fibers produce a clockwise

rotation while – in Oscillatoria princeps and Lyngbya aeruginosa the oscillin

fibers are spiraled in the opposite direction and produce a counterclockwise rotation during gliding (Fig. 2.7).

– In Phormidium, the oscillin fibers are not spiraled and the filament does not rotate during gliding.

Fig. 2.7 Rotation of the cyanobacterial filament depends on the orientation of the oscillin protein. Mucilage is secreted from the pores near the cross walls. The mucilage flows along the oscillin fibers causing rotation if the oscillin is helically oriented. There is no rotation if the oscillin is not helically oriented.

• Reversal of gliding occurs when slime stops coming out of the ring of junctional pores on one side of the septum, and when slime begins coming out of the ring of junctional pores on the other side of the septum.

Pili and twitching• Pili are proteinaceous appendages that project from the surface of

cyanobacterial cells (Fig. 2.8).• There are two types of pili in the unicellular cyanobacterium Synechocystis.

– The cell is covered uniformly with a layer of thin brush-like pili with an average diameter of 3–4 nm and a length of 1 µm.

– Cells also have thick flexible pili with a diameter of 6–8 nm and length of 4–5µm that often make connections with other cells.

• The pili are composed of 500 to 1000 units of the polypeptide pilin. Each pilin unit consists of between 145 and 170 amino acids.

• The pilin molecule is similar to the oscillin molecule involved in gliding. • Synechocystis is able to move across a surface at 1 to 2 µm s-1 using a

mechanism called twitching that utilizes change in configuration of the pili.• The pili probably move the cell body along a surface by a reiterative process

of pili extension, adhesion, and retraction.

Synechocystis cells showing pili

Sheaths• A sheath (capsule or extracellular polymeric substances (EPS))

composed of mucilage and a small amount of cellulose is commonly present in cyanobacteria.

• The sheath protects cells from drying.• Active growth appears necessary for sheath formation, a fact that may

explain its sometimes poor development around spores and akinetes.• The sheath of Gloeothece sp. is composed of polysaccharides with neutral

sugars and uronic acids including galactose, glucose, mannose, rhamnose, 2-O-methyl-D-xylose, glucuronic acid and galacturonic acids.

• The sheath of Gloeothece contains only 2% protein and a trace of fatty acids and phosphate.

• The commercial applications of cyanobacterial EPS have been reviewed by De Philippis and Vincenzini (1998).

• Sheaths are often colored, with – red sheaths found in algae from highly acid soils and – blue sheaths characteristic of algae from basic soils. – Yellow and brown sheaths are common in specimens from habitats of high salt

content, particularly after the algae dry out.

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• The sheath excludes India ink so the easiest way to visualize the sheath is to place a small amount of India ink in the water (Fig. 2.10).

• Production of a sheath is dependent on environmental conditions. – A shortage of CO2 results in cessation of sheath production and

release of the sheath. – An excess of fixed carbon results in formation of a sheath.

Protoplasmic structure• Many of the protoplasmic structures

found in the bacteria occur in the cyanobacteria.

• In the central protoplasm are the circularfibrils of DNA which are not associated with basic proteins (histones).

• The amount of DNA in unicellular cyanobacteria varies from 1.6×109 to 8.6 ×109 daltons. This is similar to the genome size in bacteria (1.0×109 to 3.6×109 daltons).

• The peripheral protoplasm is composed principally of thylakoids and their associated structures, the phycobilisomes (on the thylakoids, containing the phycobiliproteins) and glycogen granules.

• The 70S ribosomes are dispersed through out the cyanobacterial cell but are present in the highest density in the central region around the nucleoplasm.

Fig. 2.11 Drawing of the fine-structural features of acyanobacterial cell. (C) Cyanophycin body (structuredgranule); (Car) carboxysome (polyhedral body); (D) DNA fibrils; (G) gas vesicles; (P) plasmalemma; (PB) polyphosphate body; (PG) polyglucan granules; (Py) phycobilisomes; (R) ribosomes; (S) sheath; (W) wall.

• Cyanophycin is a non-ribosomally synthesized protein-like polymer that occurs in the cytoplasm in structured granules that are not surrounded by a membrane.

• Cyanophycin is a polymer that consists of equimolar amounts of arginineand aspartic acid arranged as a polyaspartate backbone and arginine side groups.

• Cyanophycin functions as a temporary nitrogen reserve in nitrogen-fixing cyanobacteria, accumulating during the transition from the exponential to the stationary phase and disappearing when balanced growth resumes (upon transfer of the maximum stationary phase culture to conditions suitable for growth).

• Nitrogen is stored in phycobilisomes in cyanobacteria that do not fixnitrogen.

Cyanophycin is composed of equimolar amountsof arginine (Arg) and aspartic acid (Asp) arranged as a polyaspartate backbone.

• Carboxysomes (polyhedral bodies) are similar to the carboxysomes in bacteria and contain the carbon dioxide-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).

• There are two types of carboxysomes, α-carboxysomesand β-carboxysomes, which differ in their protein composition. – Cyanobacteria with α-carboxysomes occur in environments

where dissolved carbon is not limiting (e.g., oligotrophic oceanic waters),

– whereas cyanobacteria with β-carboxysomes occur in environments where dissolved carbon is limiting (e.g., mats, films, estuaries, and alkaline lakes with higher densities of photosynthetic organisms).

• Carboxysomes also contain the enzyme carbonic anhydrase that converts HCO-

3 into carbon dioxide, the only form of carbon that is fixed by Rubisco (Fig. 2.15) into carbohydrates.

• The amount of a cell occupied by carboxysomes increases as the inorganic carbon (HCO-

3, CO2) in the medium decreases.• Heterocysts lack ribulose-1,5-bisphosphate carboxylase/ oxygenase

and the ability to fix carbon dioxide. Heterocysts also lack carboxysomes.

Carboxysomescontain the enzymes carbonic anhydraseand ribulose-1,5-bisphosphate carboxylase/oxygenase. Carbonic anhydrasein the carboxysomeconverts HCO3 into CO2 which is fixed by ribulose-1,5-bisphosphate carboxylase/oxygenase into carbohydrates.

• Polyphosphate bodies (metachromatic or volutin granules) are spherical and appear similar to lipid bodies of eukaryotic cells in the electron microscope. Polyphosphate bodies contain stored phosphate, the bodies being absent in young growing cells or cells grown in a phosphate-deficient medium, but present in older cells.

• Polyglucan granules (α-granules) (Fig. 2.11) are common in the space between the thylakoids in actively photosynthesizing cells. These granules contain a carbohydrate, composed of 14 to 16 glucose molecules, that is similar to amylopectin.

Polyphosphate body

polyglucan granules

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Gas vacuoles• A gas vacuole is composed of gas vesicles, or hollow cylindrical

tubes with conical ends, in the cytoplasm of cyanobacteria. • Gas vesicles do not have true protein-lipid membranes, being

composed exclusively of protein ribs or spirals arranged similarly to the hoops on a barrel.

• It is possible to collapse the gas vesicles by applying pressure to the cells, the collapsed vesicles having the two halves stuck together.

Gas vesicles. These are filled with ordinary air, and are used by cyanobacteria to regulate buoyancy in water, several of these cluster together to make a gas vacuole. The gas can occupy to 9.8% of the volume of the microbe .

• The membrane of the gas vesicle is quite rigid, with the gas inside it at a pressure of 1 atm.

• The membrane is permeable to gases, allowing the contained gas to equilibrate with gases in the surrounding solution.

• The membrane must, however, be able to exclude water.• It has been postulated that the inner surface must be hydrophobic,

thereby preventing condensation on it of water droplets, and restraining, by surface tension, water creeping through the pores.

• At the same time these molecules must present a hydrophilicsurface at the outer (water-facing) surface in order to minimize the interfacial tension, which would otherwise result in the collapse of the gas vacuole.

• Cyanobacteria possessing gas vacuoles can be divided into two physiological-ecological groups. – In the first group are those algae having vacuoles

only at certain stages of their life cycle, or only in certain types of cells. In certain species, gas vesicles appear only in hormogonia. The hormogonia float when they are released, and it is possible that the buoyancy provided is of significance in dispersal of these stages.

– The second group consists of planktoniccyanobacteria. These algae derive positive buoyancy from their gas vesicles, and as a consequence form blooms floating near the water surface.

The loss of buoyancy, and subsequent sinking of these algae in the water column, can be due to different factors.

– the loss of gas vesicles owing to increased turgor pressure, – cessation of gas vesicle production and an increase in cell mass. – entrapment of whole colonies in a colloidal precipitate composed

of organic material and iron salts. The colloidal precipitate is formed in certain lakes when dissolved iron in the anoxic water of the hypolimnion in stratified lakes becomes oxidized on mixing with aerated water of the epilimnion.

• There is a direct relationship between buoyancy and light quantity in nitrogen-fixing cyanobacteria such as Anabaena flosaquae.

• The relationship is complex and also involves the concentration of ammonium ions (NH+

4) in the water. • Buoyancy in Anabaena flosaquae increases under

– low irradiance (less than 10 µEm2 s-1), – absence of NH+

4, and – low CO2 concentrations.

• Such conditions occur in many stagnant eutrophic lakes during the summer.

• In these lakes, rapid growth of algae has depleted the NH+4 and

CO2. • The water transmits little light because of the large standing crop of

algae. • Under these conditions, A. flosaquae and other nitrogen-fixing

cyanobacteria having gas vacuoles increase their buoyancy and rise close to the surface of the water. Here they are able to out compete other algae because of their ability to fix nitrogen in water that has little available nitrogen.

• Cyanobacteria that do not fix nitrogen have reduced growth, and therefore reduced buoyancy, and sink in the water column.

• Once established, a bloom of buoyant, nitrogen-fixing, cyanobacteria tends to be self-perpetuating in that increased mass of the bloom maintains the reduced light and CO2 levels required for maximum buoyancy.

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• Structures other than gas vesicles can cause significant variation in cell density, and therefore buoyancy. – Polyphosphate granules may have a density of 2 g cm-3 or

greater, and – glycogen (which is accumulated under high light intensities) has

a density of about 1.5 cm-3.• Both have a higher density than water (1 g cm-3) and can

cause cells to sink.

Pigments and photosynthesis• The major components of the photosynthetic

light-harvesting system of the cyanobacteriaare chlorophyll a in the thylakoid membrane, and the phycobiliproteins, which are water-soluble chromoproteinsassembled into macromolecular aggregates (phycobilisomes) attached to the outer surface of the thylakoid membranes.

• Some cyanobacteria contain chlorophyll b and the cyanobacterium Acaryochlorismarina contains chlorophyll d.

• At one time those cyanobacteria containing chlorophyll b (Prochlorococcus, Prochlorothrix, Prochloron) were thought to be a distinct evolutionary group and they were placed in the Prochlorophyta.

• The absorption spectrum of chlorophyll d is shifted toward far-red wavelengths and A. marina exists in environments where there is an abundance of these wavelengths of light (e.g., under red algae).

• The carotenoids of the cyanobacteria differ from those of the eukaryotic algae– in having echineone (4-keto-β -carotene) and

myxoxanthophyll, which eukaryotic algae do not have; – in lacking lutein, the major xanthophyll of chloroplasts; and – in having much higher proportions of β–carotene than are found

in eukaryotic algae.

• The Cyanophyceae have four phycobiliproteins:– C-phycocyanin (absorption maximum at a wave length [λ] of 620 nm), – allophycocyanin (λmax at 650 nm), – C-phycoerythrin (λmax at 565 nm), and – phycoerythrocyanin (λmax at 568 nm).

• All cyanobacteria contain the first two, whereas C-phycoerythrin and phycoerythrocyanin occur only in some species.

• The phycobiliproteins of the cyanobacteria change in concentration in response to light quality and growth conditions.

• Cyanobacteria that produce the red phycoerythrin and the blue phycocyaninin white light, suppress phycoerythrin synthesis in red light and phycocyaninsynthesis in green light (complementary chromatic adaptation).

• For example, Oscillatoria sancta assumed a blue-green tint after growth under orange light (due to suppression phycoerythrin synthesis) and a red coloration after growth under green light (due to suppression phycocyaninsynthesis).

Complementary chromatic adaptation is a spectacular response of some blue-green and red algae to alterations in the energy distribution in the visible light environment .

• It is believed that the thylakoids in cyanobacteria were probably originated by invaginations of the plasmalemma; some cyanobacteria today have thylakoids that are continuous with the plasmalemma.

• An example of the primitive condition may be Gloeobacter violaceus, a unicellular cyanobacteria that lacks thylakoids but has chlorophyll a, carotenoids, and phycobiliproteins.

• In this alga the pigments, and presumably photosynthesis, are associated with the plasmalemma.

Gloeobacter violaceus(Phy) Probable layer of phycobiliproteins(Pl) plasmalemma

• Many cyanobacteria have the ability to photosynthesize under aerobic or anaerobic conditions.– Under aerobic conditions, electrons for photosystem I are derived from

photosystem II. – Under anaerobic conditions, in the presence of sulfur, electrons are

derived by the reduction of sulfur:

• These cyanobacteria are facultative phototrophic anaerobes and fill an important ecological niche in aquatic systems.

• Eukaryotic algae are restricted to photoaerobic habitats, whereas photosynthetic bacteria are restricted to photo-anaerobic habitats.

• In habitats that fluctuate between the above conditions, cyanobacteriawith facultative anaerobic photosynthesis have a clear selectiveadvantage.

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– An example of this is the Solar Lake, Eilat, Palestine, where in the winter high levels of sulfide are found in the anaerobic bottom layers of water of the thermally stratified lake. Oscillatorialimnetica occurs in these highly anaerobic bottom layers, where sulfide functions as an electron donor for photosynthesis.

– In the spring, the lake overturns, with all of the water becoming aerobic, Oscillatoria limnetica activates aerobic photosystem II and carries out photosynthesis aerobically.

– Thus O. limnetica, by utilizing combined anoxygenic and oxygenic photosynthesis, is the dominant phototroph of the Solar Lake, with its fluctuating photoaerobic and photoanaerobicconditions.

• Photosynthesis in many cyanobacteria is stimulated by lowered oxygen concentration, the oxygen competing with carbon dioxide for the enzyme ribulose-1, 5-bisphosphate carboxylase/ oxygenase.

• This phenomenon probably reflects an adaptation to the absence of free oxygen in the atmosphere of Precambrian times when the cyanobacteria first evolved.

• After the evolution of the oxygen-evolving cyanobacteria, the oxygen in the atmosphere gradually built up, creating a protective ozone (O3)layer in the atmosphere at the same time. The ozone layer removed most of the harmful ultraviolet radiation from the sun and allowed the evolution of more radiation sensitive organisms.

• The cyanobacteria are relatively insensitive to radiation, having a system that repairs radiation damage.

Fig. 2.21 Graph showing stimulation of photosynthesis (●-●) and respiration (■-■) by low concentrations of atmospheric O2 in Anabaena flos-aquae.

• Akinetes are generally recognized by their larger size relative to vegetative cells and conspicuous granulation due to high concentrations of glycogen and cyanophycin.

• The most consistent property of akinetes is their greater resistance to cold compared with vegetative cells.

• Akinetes have often been compared to endospores in Gram-positive bacteria.

• Akinetes, however, are neither as metabolically quiescent nor as resistant to various environmental extremes.

• Akinetes only occur in cyanobacteria that form heterocysts.

Light micrographs of akinetes in Raphidiopsis mediterranea

Electron micrograph of a mature akinete of Cylindrospermum sp. with a thick layered wall (f,l) and cytoplasm full of proteinaceous cyanophycin (structured) granules (s), polyhedral bodies, and ribosomes.

• In Aphanizomenon, the development of akinetes from vegetative cells involves (1) an increase in cell size, (2) the gradual disappearance of gas vacuoles, and (3) an increase in cytoplasmicdensity and number of ribosomes and cyanophycin granules.

• Akinetes of Nostoc lose 90% of their photosynthetic and respiratory capabilities, as compared with vegetative cells. The loss occurseven though there is little change in phycocyanin and chlorophyll, the main photosynthetic pigments.

• Mature akinetes are usually considerably larger than vegetative cells, contain protoplasm full of food reserves, and have a normal cell wall surrounded by a wide three-layered coat.

• Loss of flotation by an increase in cytoplasmic density causes filaments with akinetes to sink and overwinter in bottom sediments.

• In akinete germination, there is a reverse of the above events (Fig. 2.24).

• A wide range of physicochemical factors have been reported to stimulate akinete differentiation; for example, phosphate deficiency, low temperature, carbon limitation and reduction in the availability of light energy.

Fig. 2.24 The germination of an akinete of Cylindrospermopsis raciborskii.

Heterocysts• Heterocysts are larger than vegetative cells and appear empty in

the light microscope (whereas akinetes appear full of storage products).

• Heterocytes can be terminal as in Gloeotrichia, or intercalary as in Anabaena.

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• Heterocysts have been drastically altered to provide the necessary anoxic environment which is ideal for nitrogenase, a notoriously O2sensitive enzyme necessary for the process of nitrogen fixation as following:– They are photosynthetically inactive because they loss of photosystem

II thus they do not fix CO2, nor do they produce O2.– They also exhibit a high rate of respiratory O2 consumption– They are surrounded by a thick, laminated cell wall that limits ingress of

atmospheric gases, including O2.– Heterocysts also have a form of myoglobin called cyanoglobin that

scavenges oxygen, preventing inhibition of nitrogenase

Fig. 2.25 Three-dimensional view of a heterocyst. The envelope has homogeneous (H), fibrous (F), and laminated (L) layers. (M) Membranes; (P) pore channel; (Pl) plasmalemma; (W) cell wall.

• Heterocysts are formed at regular intervals from vegetative cells by

– the dissolution of storage granules,

– the deposition of a multilayered envelope outside of the cell wall,

– the breakdown of photosynthetic thylakoids,

– and the formation of new membranous structures.

• The differentiation of heterocysts in Anabaena is triggered by nitrogen deprivation (ammonia, nitrate, nitrite) in two steps: the first step is reversible and the second step is irreversible.

Fig. 2.26 Summation of events leading to the formation of heterocysts.

• Commitment point 1.• 2-oxoglutarate (α-

ketoglutarate) is the substrate used for incorporation of ammonium in cyanobacteria.

• The absence of combined nitrogen leads to an increase in intracellular 2-oxoglutarate since cyanobacteria lack 2-oxoglutarate dehydrogenaseand the ability to breakdown 2-oxoglutarate.

• The increase in intracellular 2-oxoglutarate activates the protein NtcA and results in a drop in the amount of the calcium-binding protein Ccbp, and a rise in Ca2+ in the cells.

• Heterocysts have 10 times more Ca2+ than vegetative cells.

α-Ketoglutaric acid

• The rise in Ca2+ up-regulates the hetR gene that forms hetR, a serine-type protease, which induces the vegetative cell to change into a heterocyst.

• The hetR protein is considered the “master switch” in heterocyst development.

• The production of hetRprotein constitutes commitment point 1.

• A lack of combined nitrogen over a period of about 10 hours results in continued production of hetR protein and the cell becoming a proheterocyst, which under the light microscope appears less granulated than vegetative cells but still lacks a thick cell wall.

• The hetR protein also induces the production of an oligopeptide called PatS.

• This oligopeptide diffuses to adjacent cells where it prohibits the formation of hetR protein in the cells and assures that the adjacent cells do not transform into proheterocysts.

• This sets up the spacing seen in Anabaena where heterocysts are formed equidistant from each other.

• Commitment point 1 is reversible. A proheterocystwill revert back to a vegetative cell if combined nitrogen is added to the medium, causing a switch“off” of the hetR gene and formation of hetR protein.

• Differentiation of a vegetative cell to a proheterocyst also involves loss of photosystem II activity, thus eliminating photosynthetic generation of O2 (which inhibits nitrogen fixation).

• Commitment point 2.• The second stage can not

be reversed by the addition of combined nitrogen and comprises the transformation of a proheterocyst into a mature heterocyst.

• This involves the formation of a thick cell wall containing glycolipids and polysaccharides to reduce diffusion of O2.

• The nitrogen-fixing enzyme nitrogenase is activated when 11 and 55 kilobasesare removed between repeat sequences flanking nitrogenase (nif ) genes.

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• Rearrangement of nitrogen fixation (nif) genes in the heterocysts of Anabaena sp. • 11 and 55 kb elements are excised and remain in heterocysts as unreplicated and untranscribed

circles.• As a consequence of the 11kb and 55 kb elements removal from the chromosome a functional nif

Operons are created and Nitrogenase proteins are then expressed from the rearranged genes in heterocysts and aerobic nitrogen fixation ensues.

• It has been shown that the lack of nitrogen in the medium increases the number of heterocysts, whereas nitrogen-rich media inhibit their development.

• Since akinetes and heterocysts share a unique properties, It is hypothesized that akinetes were the evolutionary precursors of heterocysts.

• Akinetes are only seen in heterocyst-forming cyanobacteria, akinetes contain glycolipids characteristic of heterocysts, and the cell wall of heterocysts is identical to that of akinetes (and unlike that of vegetative cells).

• Apart from the exceptional cases of germination, heterocysts are unable to divide.

• Heterocysts have a limited period of physiological activity and appear to have a limited life.

• Senescent heterocysts undergo vacuolation and usually break off from the filament, causing fragmentation of the filament.

• Heterocysts are dependent on a supply of substrates from adjacent vegetative cells through cytoplasmic connections (microplasmodesmata).

• These cytoplasmic connections probably convey nitrogen fixed in the form of glutamine by the heterocysts to vegetative cells.

• The vegetative cells transfer photosynthate to the heterocysts since the heterocysts are incapable of carbon fixation.

Nitrogen fixation• Cyanobacteria are diazotrophs (able to fix atmospheric nitrogen).• In nitrogen fixation, N2 is fixed by the enzyme nitrogenase into ammonium

using ATP as a source of energy (Fig. 2.27).• Nitrogenase, the nitrogen-fixing enzyme, is very sensitive to inactivation by

oxygen. Cyanobacteria have evolved three different mechanisms designed to exclude oxygen from the area of the cells containing nitrogenase:

Fig. 2.27 The chemistry of nitrogen fixation and the subsequent incorporation of the fixed nitrogen into glutamate and glutamine.

• 1 Heterocystouscyanobacteria: These cyanobacteria occur primarily in fresh and brackish water and fix nitrogen in heterocysts(described before).

• Under anaerobic conditions, in an atmosphere of nitrogen and carbon dioxide, both vegetative cells and heterocysts can fix nitrogen.

• 2 Non-filamentous cyanobacteria that fix nitrogen in the dark but not in the light:These cyanobacteria fix nitrogen in the dark when photosynthesis is not producing nitrogenase-inhibiting oxygen.

• If Synechococcus is grown under a 12-hour light: 12-hour dark cycle, most of the nitrogen fixation occurs during the dark period (Fig. 2.30).

• At the beginning of the light period, oxygen is produced by photosynthesis and the nitrogenase is inactivated. New nitrogenase must be synthesized at the beginning of the next dark period for nitrogen fixation to occur.

Fig. 2.30 Illustration of the relationship of between photosynthesis and nitrogen fixation in a culture Synechococcus sp. growing under a 12-hour light: 12-hour dark photoperiod. Under continuous illumination, an endogenous clock maintains the cycles.

• 3. Trichodesmium and Katagnymene: These cyanobacteria are the major bloom-forming, nitrogen-fixing, organisms in the oceans, responsible for fixing one-quarter of the total nitrogen in the oceans of the world.

• These filamentous cyanobacteria do not have heterocysts yet fix nitrogenin the light under aerobic conditions.

• Within the filaments, 10 to 15% of the cells (called diazocytes) are specialized to fix nitrogen, while the others do not.

• Cells that fix nitrogen are adjacent to one another and have a denser thylakoid network with fewer gas vacuoles and cyanophycin granules.

• The tropical seas where Trichodesmium and Katagnymene live are relatively low in dissolved oxygen and this may assist the nitrogen-fixing cells in maintaining anaerobic conditions in the protoplasm where nitrogenase is present.

• Trichodesmium and Katagnymene represent the most ancient type of nitrogen fixing cyanobacteria.

Some nitrogen-fixing cyanobacteria that lack heterocysts.

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Circadian rhythms

• The cyanobacteriahave circadian rhythms in photosynthesis, nitrogen fixation, and cell division similar to those in eukaryotic organisms (Fig. 2.30).

Asexual reproduction

• Asexual reproduction occurs by the formation of hormogonia or baeocytes or fragmentation of colonies

(a),(b) Formation of a hormogonium in Oscillatoria. (c) Baeocyte formation in Chamaesiphon incrustans. (d) Baeocyte formation in Dermocarpa pacifica.

• Hormogonia (or hormogones), which are characteristic of all truly filamentous cyanobacteria, are short pieces of trichome that become detached from the parent filament and move away by gliding, eventually developing into a separate filament.

• The one common factor in initiation of hormogonium differentiation appears to be a change in some environmental parameter such as an increase or decrease of a nutrient or a change in the quantity of light.

Fig. 2.35 Nostoc punctiforme. (a) Vegetative cells. (b) Hormogonia. The hormogonialack heterocysts and are smaller than vegetative cells.

• Baeocytes (endospores) which literally means "small cell" are formed by some coccoid (spherical) cyanobacteria. The protoplasm divides several times in different planes without growth between successive divisions.

• The resulting baeocytes are smaller than the original cell. Baeocytesare similar to bacterial endospores. In Dermocarpella, the baeocytesare released through an apical pore and enlarge to mature organisms

Top view showing radial divisions.

Scanning electron micrograph showing apical pore through which baeocytes escape.

Growth and metabolism• In the cyanobacteria there are three nutritional types: • (1) facultative chemoheterotrophs, or those organisms capable of

growing in the dark on an organic carbon source and of growing phototrophically in the light (only a portion of the cyanobacteriaexhibit this condition). Facultative chemoheterotrophs are able to grow in the dark on a very narrow range of substrates, being confined to glucose, fructose, and one or two disaccharides.

• (2) obligate phototrophs, or organisms that can grow only in the light on an inorganic medium (some of these are actually auxotrophs, requiring a small amount of an organic compound that is not used as a source of carbon, invariably meaning a vitamin);

• (3) photoheterotrophs, or those cells that are able to use organic compounds as a source of carbon in the light but not in the dark.

Lack of feedback control of enzyme biosynthesis• Cyanobacteria, such as Chlorogloea fritschii (Fig. 2.37), lack metabolic

control of many pathways by repression and derepression of enzyme biosynthesis as occurs in many other organisms.

• In other organisms, as an end product of a pathway builds up to excess, the end product represses an enzyme involved in its synthesis, thereby directing precursors to other parts of the cell’s metabolism, where they can be better used. When the amount of end product falls, the enzyme is derepressed, allowing manufacture of the end product again.

• Cyanobacteria often release considerable quantities of nitrogenous and organic substances into the medium, with most of the compounds being excreted as peptides. Such an excretion, could be a necessary consequence of ill-controlled amino acid biosynthesis. Unable accurately to adjust the synthesis of each amino acid to the needs of protein synthesis, the cells would inevitably have to synthesize an excess of amino acids to ensure that protein synthesis would proceed smoothly. This excess would then form the basis of the extracellular peptide material associated with the cyanobacteria.

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Symbiosis• The cyanobacteria occur in basically two types

of associations: – those in which the cyanobacterium is extracellular

and – those in which it is intracellular.

Extracellular associations• In extracellular associations all Cyanobacteria show a decrease in growth,

assimilation of CO2 and assimilation of NH+4.

• Conversely, there is an increase in the rate of N2 fixation that accompanies the increased numbers of heterocysts.

• The heterocysts and vegetative cells of Nostoc within leaf cavities of Azollaare up to fourfold larger than the vegetative cells of free-living cyanobacteria.

• In the symbiosis, Nostoc receives hexose sugars from the host, while Nostocprovides fixed nitrogen to the host.

• There is also a five- to ten-fold increase in the production of motile hormogonia that serve as infective units in the establishment of the symbioses.

Azolla leaf with Anabaena in gas chambe Nostoc-(Anabaena)-azollae Azolla

• The water fern Azolla (Fig. below) has cavities in the dorsal lobe of the leaf that are occupied by Anabaena azollae.

• This cyanobacterium fixes nitrogen, some of which is excreted into the cavity and taken up by the cells of the Azolla.

• The Anabaena is apparently unable to live outside of the host.• Nitrogen fixation by the cyanobacterial symbionts of Azolla has for

many years been utilized to advantage in the Far East; here the water fern has been used as a green manure in rice fields, where about 3 kg of atmospheric nitrogen per hectare per day is fixed by Azolla symbionts

Longitudinal sections through the dorsal lobe of a mature (left) and an immature (right) leaf of the water fern Azolla illustrating filaments of Anabaena in leaf cavities.

• Azolla is a symbiotic superorganism that captures all the nitrogen fertilizer it needs to grow from the air around it. Asia’s farmers have long known this, growing Azolla together with rice to provide a natural fertilizer to bolster rice productivity .

• Cycad roots are frequently infected with cyanobacteria that cause distortions of the roots (coralloid roots) (Figs. a, b). In these roots the cyanobacteria occupy a clearly defined cortical zone midway between the pericycle and the epidermis. The cyanobacteria occur in the intercellular spaces and are surrounded by a sheath and amultilayered wall, whereas the adjacent cycad cells have a reduced chromosome number and secrete slime. These cyanobacteria are able to fix nitrogen and contribute a portion of the fixed nitrogen to the cycad cells

(a) Root nodules of the cycad Dioon spinulosum. (b) Part of a longitudinal section of a nodule of Encephalartos with an algal zone.

lichens

Fig. 2.38 Lichina pygmea is a lichen containing a cyanobacteria as the phycobiont. Lichinaoccurs on rocks near the high-tide mark and looks like a stubby seaweed with its club-shaped branches making a sward of uniform height. The cyanobacterial phycobiont transfers glucose to the mycobiont which converts the glucose to mannitol.

• The most common type of extracellular association is that with fungi to form lichens.

• In most lichens, there is a single algal component (phycobiont), which is a green alga, cyanobacterium, or, as with the fungi species Verrucaria, a member of the Xanthophyceae.

• Cyanobacteria occur in about 8% of the species of lichens (Fig. 2.38).• Almost all of the fungal partners (mycobionts) are ascomycetes, but certain

imperfect fungi and basidiomycetes also occur in lichens. • Because the mycobionts frequently reproduce sexually whereas the

phycobionts usually do not, the lichens are classified with the fungi. • In the symbiotic association, the phycobiont fixes carbon in photosynthesis

and liberates it as glucose, which the mycobiont converts into mannitol and assimilates (Fig. 2.38).

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• Approximately 40% of the photosynthate of the alga is secreted as glucose in the lichen association.

• Whereas the benefits to the fungus in the association are obvious, the benefits to the alga are probably limited to some protection against desiccation.

Intracellular associations• Intracellular associations involving

– cyanobacteria are usually more specialized than extracellularassociations,

– and it is not possible to culture the cyanobacteria away from the host.

• The term cyanelle is used for intracellular cyanobacteriain symbioses, and the term cyanome for the host cell.

• It is probable that one of the endosymbiotic associations led to the development of chloroplasts in some of the algal groups.

• Colonies of the coral Montastraea cavernosa contain endosymbiotic coccoidcyanobacteria in vacuoles in the epithelial cells of the coral. The cyanobacteria are capable of nitrogen fixation. Anaerobic conditions (necessary for function of the nitrogen-fixing enzyme nitrogenase) are created by the Mehler reaction where glycerol provided by dinoflagellatezooxanthellae is reduced.

Mehler reactionThe dinoflagellate zooxanthellae produce glycerol which is transferred to the cyanobacteria. Glycolytic breakdown of glycerol produces hydrogen atoms that reduce oxygen molecules produced in photosynthesis. This creates the hypoxia necessary for nitrogenase activity and nitrogen fixation.

Montastraea cavernosa

Ecology of cyanobacteria

Marine environmentLittoral zone• Cyanobacteria in the littoral zone occur as a black encrusting film on rocks

at the upper limit of the high-tide mark. Most of the cyanobacteria in the littoral zone are nitrogen fixing, and they make a significant contribution to the productivity of rocky shores and coral reefs.

Open ocean cyanobacteria• In the open ocean, most of the total photosynthetic capacity is made up of

picophytoplankton (phytoplankton cells unable to pass through a filter with 2-µm-diameter holes), made up principally of tiny coccoid cyanobacteria at concentrations of around 10 000 cells per milliliter.

• The picophytoplanktonic cyanobacteria are more evident in nutrient-poor offshore waters where larger phytoplankton are less successful, because they are less able to utilize low concentrations of nutrients.

• Cyanobacteria larger than picophytoplankton often form a significant part of oceanic phytoplankton.

• Massive development of filaments of the nitrogen-fixing Trichodesmium occurs in certain tropical waters.

• Each colony of Trichodesmium consists of a mass of filaments that secrete a flocculent mucilage which supports bacterial colonies; these in turn are fed on by different protozoa. The large surface area produced by the algal filaments forms a miniature ecosystem

• Trichodesmium produce gas vacuoles which, under calm conditions, cause the cells to accumulate at the water surface, giving rise to the phenomenon known to sailors as “sea sawdust,” or long orange or gray windrows of algae.

• One such bloom stretched 1600 km along the Queensland coast of Australia, extending from the shore to the Great Barrier Reef, and occupying an area of 52 000 km2.

• Trichodesmium also occurs in the Red Sea and it was probably the color produced by blooms of the alga that gave the Red Sea its name.

• Trichodesmium moves up in the water column by means of gas vesicles, and moves down in the water column by carbohydrate ballasting. The cells become progressively heavier from morning to evening as carbohydrates and polyphosphate bodies are produced.

• In the Caribbean, Trichodesmium cells have been found down to 200 m. The gas vesicles of this cyanobacteriumare much stronger and more difficult to collapse than those found in any freshwater alga. The gas vesicles are able to stand up to 20 atm of pressure, enabling Trichodesmium to rise from great depths.

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Freshwater environment• Freshwater blooms of cyanobacteria are common. Most freshwater

blooms of cyanobacteria consist of Microcystis, Anabaena, Aphanizomenon, Gloeotrichia, Lyngbya or Oscillatoria.

• Although they occur in lakes over the whole year, it is usually only in late summer and early autumn that they reach bloom proportions.

• This because of:1 the superior light-capturing abilities of cyanobacteria when self-

shading is the greatest.2 their high affinity for nitrogen and phosphorus when nutrient limitation

is most severe.3 their ability to regulate their position in the water column by gas

vacuoles to take advantage of areas richer in nutrients and/or light.4 their higher temperature optima for growth and photosynthesis

(greater than 20 °C).

Hot-spring cyanobacteria• Cyanobacteria are important in the colonization of non-acidic hot

springs through out the world.• Some cyanobacteria have the ability to grow at temperatures as high

as 70 to 73 °C, a much higher temperature tolerance than occurs in eukaryotic algae.

• In thermal environments above 56 to 60 °C, both photosynthetic and nonphotosynthetic eukaryotic algae are always absent.

• In acid springs (pH less than 4), no cyanobacteria are present, and at temperatures above 56 °C in these springs there are no photosynthetic organisms at all.

• The thermophilic cyanobacteria are especially adapted to live at elevated temperatures. Enzymes isolated from thermal algae are more stable at higher temperatures than those from other organisms.

Terrestrial environmentTerrestrial cyanobacteria play a major role as primary

colonizers in the establishment of a soil flora and in the accumulation of humus.

They do this in four main ways:1. They bind sand and soil particles and prevent erosion.

They do this with their gelatinous sheaths and by their growth pattern which produces closely intertwined rope-like bundles in and among soil particles.

2. They help to maintain moisture in the soil. Booth (1941), found that soil with an algal covering had a moisture content of 8.9% compared to 1.3% in the absence of algae.

3. They are important as contributors of combined nitrogen through nitrogen fixation. In grass lands, the soil surface between crowns of grasses may support extensive zones of cyanobacteria and lichens that include cyanobacteria as their phycobiont constituting up to 20% of the ground cover.

4. It has been suggested that cyanobacteria assist higher plant growth by supplying growth substances.

• Anhydrobiotics are organisms that can withstand the removal of the bulk of their intracellular water for extended periods of time.

• The cosmopolitan terrestrial cyanobacterium Nostoc commune is able to tolerate acute water stress and can survive in the air-dry state for many years.

• Approximately 0.1 g of blackened air-dried colonies becomes an olive-green rubbery mass of more than 20 g wet weight within 30 min of rehydration.

• In Nostoc commune, rehydration rather than desiccation appears to be the fatal event.

• To protect the cells during rehydration, a water stress protein and large amounts of the sugar trehalose are synthesized that stabilize the phospholipid bilayers of cellular membranes

Nostoc commune.

• Cyanobacteria comprise the dominant component of the soil photosynthetic community in hot and cold arid regions where higher plant vegetation is absent or restricted.

• Desert cryptobiotic crusts (also called biological soil crust) are initiated by the growth of cyanobacteria in the soil during episodic events of available moisture.

• The only cyanobacteria that are initial colonizers are those that – have heterocysts, and are therefore able to fix nitrogen; – and those cyanobacteria that produce scytonemin, a sunscreen

that accumulates in the cyanobacterial sheaths and absorbs some of the strong sunlight in the near ultraviolet (370–384 nm)

• Microcoleus vaginatus (Fig. 2.45) makes up over 90% of cryptobioticcrusts in the arid soil of the Colorado Plateau in the United States.

• Filaments of M. vaginatus are surrounded by thick mucilaginous sheaths that can absorb eight times their weight in water, increasing the water capacity of sandy soils.

• Clay particles and sand grains become trapped in the cyanobacterialsheaths (Fig. 2.45).

• The clay particles are negatively charged and bind positive cationic nutrients (e.g., K+, Ca+2) preventing them from leaching into the subsoil.

• Subsequently, lichens, fungi and moss establish themselves in the crust, enriching and stabilizing the soil.

Scanning electron micrographs of Microcoleus vaginatus. Left: Filament. Right: Filament with clay particles and sand grains embedded in the slime surrounding the cell walls.

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• From the above discussion, it is plain that cyanobacteriaoccur in most environments on earth. There is, however, one important exception:

cyanobacteria are usually absent in waters or soils with a pH below 5, and they are uncommon between pH 5 and 6

Adaption to silting and salinity• In salt marshes and mud flats, Microcoleus chthonoplastes are phototactic

and chemotactic, resulting in migration to the surface of the mud at a rate of about 7 mm per 24 h. The movement of the filaments is an adaption that allows the cyanobacterium to survive during silting.

• M. chthonoplastes is euryhaline, it is able to survive in a wide range of salinities in the estuarine environment by producing glycosylglycerol as an osmolyte to counteract the osmolarity of the surrounding environment.

• It also synthesizes trehalose to stabilize the phospholipid membrane bilayers of the cell

• In hypersaline environments, halotolerant and halophiliccyanobacteria can adapt to the high salinity in three ways

1. Active export (influx) of inorganic ions that steadily diffuse along their electrochemical gradients into the cytoplasm leading to relatively unchanged internal salt concentrations.

2. Accumulation of organic osmoprotective compounds, such as glucosylglycerol, glycine, and betaine (trimethylglycine), to maintain the osmotic equilibrium.

3. Expression of a set of salt-stress proteins such as the protein flavodoxin.

Cyanotoxins• Some of the cyanobacteria produce toxins (cyanotoxins). • Physiologically, there are basically two types of cyanotoxins:

neurotoxins and hepatotoxins.• Neurotoxins – The neurotoxins are alkaloids (nitrogen-containing

compounds of low molecular weight) that block transmission of the signal from neuron to neuron and neuron to muscle in animals andman. Symptoms include staggering, muscle twitching, gasping and convulsions.

• The neurotoxins can be fatal at high concentrations due to respiratory arrest caused by failure of the muscular diaphragm.

• The two neurotoxins produced by cyanobacteria are anatoxin and saxitoxin. Anatoxins are synthesized by species of Anabaena, Aphanizomenon, Oscillatoria and Trichodesmium.

• Hepatotoxins – The heptatotoxins are inhibitors of protein phosphatases 1 and 2A and affect the animal by causing bleeding in the liver.

• Clinical signs include weakness, vomiting, diarrhea, and cold extremities.

• Cyanobacteria produce two types of hepatotoxins, the microcystinsand nodularins, that are produced along a similar pathway.

• Microcystins are synthesized by species of Microcystis, Anabaena, Nostoc, Nodularia, and Oscillatoria while the nodularins are produced by species of Nodularia.

• The cyanotoxins are mostly important in freshwaters where the cyanobacteria are ingested in drinking water by animals, with the algae dying and releasing their toxins in the intestinal tracts.

• The cyanotoxins are responsible for the loss of large numbers of stock each year through out the world, usually in the warm summer months when the blooms of cyanobacteria are visible in the water.

• Seldom will man drink from such an unattractive water source. Assuch, poisoning of man by cyanotoxins is relatively rare, partly because of the odor from geosmin and MIB (Fig. 2.50) produced by cyanobacteria.

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• The cyanotoxins function as anti-herbivore chemicals by inhibiting invertebrate grazers in the aquatic environment. Grazing by invertebrates causes Microcystis aeroginosa to increase production of cyanotoxin.

• Cyanotoxins also can inhibit the growth of other algae. This is called an allelopathic interaction where one organism affects the growth of a second organism. An example of this is the inhibition of the freshwater dinoflagellate Peridinium gatunense by the microcystinproduced by the cyanobacterium Microcystis in Lake Kinneret, Palestine.

• The Microcystis cyanotoxins abolish carbonic anhydrase activity in the dinoflagellate and inhibit growth.

• The microcystin is classified as an allelochemical and acts as an algicide.

Microcystis aeruginosa

• Another allelopathic interaction involves the zebra mussel (Dreissena polymorpha) and the cyanobacterium Microcystis.

• Zebra mussels, native to the Black and Caspian seas, were discovered in the United States near Detroit in 1988. Since thenzebra mussels have spread throughout the Great Lakes of North America, displacing native shellfish. The zebra mussel has been beneficial in low-nutrient lakes (but not in lakes high in nutrients) because they graze on Microcystis, probably because the mussels have nothing else to eat (Raikow et al., 2004).

• https://www.kbs.msu.edu/images/stories/docs/hamilton/raikow.pdf

Cyanobacteria and the quality of drinking water• Geosmin (E-1,10-dimethyl-E-9-decalol) and MIB (2-

methylisoborneol) are two terpenoids (isoprenoids) that are produced by some cyanobacteria.

• These terpenoids are called volatile organic compounds (VOCs), have a highly potent earthy, musty or muddy aroma and account for most of the odors in drinking water. They have odor threshold concentrations of ~10 ng l-1.

• Terpenoids as a class have extensive olfactory properties, which are exploited commercially in the food, beverage, and perfume industries. Both geosmin and MIB resist conventional water treatment, and their bioaccumulation in fish and shellfish causes off-flavor in farmed and wild stocks.

• Neither geosmin nor MIB are toxic to vertebrates (including humans).

• The low incidence of human poisonings by cyanotoxins has been attributed to the avoidance of water containing cyanobacteriabecause of the odors produced by geosmin and MIB.

• Both compounds are commonly found in freshwater and solid habitats, but are rare in offshore marine environments, and can be used as landmass indicators.

• These volatile organic compounds produced by cyanobacteria act as infochemicals by attracting nematodes to cyanobacterial colonies where the nematodes feed and deposit eggs.

Utilization of cyanobacteria as food• Cyanobacteria are used as human food and animal food

supplements. In China, the Spirulina industry is supported by the State Science and Technology Commission as a natural strategic program. In 1996 there were more than 90 Spirulina factories with a total production of 400 tons of Spirulina dry powder and a total production area of 1 million square meters.

• Besides Spirulina pills and capsules, there are also pastries, blocks, and Spirulina-filled chocolate blocks.

• Nostoc is a cyanobacterium that has been gathered for food for over two thousand years in China. It is called “hair vegetable” because of its hair-like appearance. The Chinese word for “hair vegetable”(Facai) sounds like another Chinese word that means fortunate and get rich, adding a spiritual value to the cyanobacterial food.

• In Japan, Aphanothece sacrum, Nostoc verrucosum, N. commune, and Brachytrichia have been used as side dishes since ancient time.

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• Aphanizomenon flosaquae has been harvested from Klamath Lake in California since the early 1980s and sold as a food and health supplement. In 1998, 10 ×106 kg (dry weight) was marketed with a value of $100 million (Carmichael et al., 2000).

• Other cyanobacteria, such as Spirulina, are readily available at health-food stores in the developed world, although one wonders how healthy these cyanobacteria really are in regard to the potential for this group of algae to produce cyanotoxins.

Spirulina major

Cyanophages• Cyanophages are viruses that infect and commonly kill

cyanobacteria.

• Most cyanobacteria are actually resistant to attack by cyanophageswith the cyanophage population being maintained by infection of the relatively rare cyanobacteria that are susceptible to infection.

• Cyanobacteria in the open ocean are more susceptible to infection than cyanobacteria from inshore waters.

• High temperature and phosphate limitation increase the probably of cyanobacterial infection by cyanophages.

Secretion of antibiotics and siderophores• Some cyanobacteria, such as Nostoc secrete antibiotics called

bacteriocins, which kill related strains of the alga. • A bacteriocin is a proteinaceous antibiotic that is active against

prokaryotic strains closely related to the organism that produces the antibiotic.

• Other cyanobacteria secrete antibiotics that are active against a wide range of cyanobacteria and eukaryotic algae.

• Scytonema hofmanni (Fig. 2.46(e)) produces such an antibiotic. This antibiotic, called cyanobacterin, is a chlorine-containing λ–lactone.

• All of these antibiotics probably play an active role in the survival of the producing organism by inhibiting growth of competing organisms.

Scytonema hofmanni

• The obligate requirement for iron, coupled with the low solubility of iron in many aquatic habitats, has led to the evolution of a mechanism for iron acqusition, at the cost of cellular energy and nutritional stores.

• A number of cyanobacteria release extracellular ferric-specific chelating agents (“siderophores”) during periods of low iron availability.

• Siderophores function as extracellular ligands that complex with iron ions Fe3+ in the medium. The siderophore complexed iron ions are absorbed by the cell (Fig. 2.52)

Fig. 2.52 (a) A siderophore complexed with two ions of iron. (b) A cyanobacterial cell secretes a siderophore which complexes with iron ions in the medium. The siderophore complexed iron ions are absorbed by the cell.

Secretion of siderophores

Calcium carbonate deposition andfossil record

• Many species of cyanobacteria have calcium carbonate in the enveloping mucilage of the cells.

• In fresh water these algae usually grow in water where carbonatecrystallizes out by non-biological physicochemical mechanisms, and the crystals of calcite become trapped in the mucilage of the algae. Normally, only 1% to 2% of the calcium carbonate is actively precipitated by the cells.

• There are a large number of different forms of carbonate deposits attributed to the Cyanophyceae

• In the marine environment, Stromatolithic heads are probably the best known of these forms.

• These heads are firmly gelatinous to almost cartilaginous in texture, predominantly hemispherical, and show fine concentric lamination.

• During the day there is growth of algal filaments, resulting in the algae covering the surface of the stomatolite head.

• During the night, growth ceases, and sediments accumulate on the surface of the head, forming sediment-rich laminae up to 100 µm thick.

• In the early part of the day, the algae penetrate through this deposited sediment, and grow a hyaline layer, 200 µm thick, with a low concentration of entrapped sediment.

• These alternating periods of growth and deposition give a laminated structure to the stromatolite.

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• The production of laminae in stromatolites depends on fluctuations ultimately derived from the physical movements of the earth, sun, and moon, and requires some kind of rhythmicity that causes discontinuity in the accretionary process.

• The periodicity of laminae is due primarily to the daily photosynthetic cycle of the organisms in the stromatolites.

• In addition, stromatolites are heliotropic and grow toward sun light. This heliotropism allows the calculation of the extent of a year’s deposition in a stromatolite.

• The yearly cycle of movement of the sun causes the sun to be higher in the sky in the summer and lower in the winter.

• The heliotropism of the stromatolites causes the stromatolites to grow in a sine waveform over the course of a year (Fig. 2.55).

• Counting the number of laminae in one sine wave has allowed paleontologists to calculate the number of days in a year for a geological period.

• Such studies have shown that the solar year has varied considerably. For example, approximately 1000 million years ago, the solar year consisted of approximately 435 days.

Fig. 2.55 Diagrammatic representation of the growth of a stromatolite over the period of a year. A year’s growth is represented by an S-shaped curve.

• Up to 2000 million years ago, there were no grazing and boring organisms; thus stromatolites grew uncontested. Without competition, Precambrian stromatolites freely populated enormous areas and most likely grew in water down to a depth of 10 m.

• The occurrence and size of stromatolites declined dramatically after the evolution of grazing and boring organisms.

• Today stromatolites grow only in warm waters that are inhospitable to grazers and borers – waters such as the hypersaline waters in Shark Bay, Australia, or in waters with a high tidal current that inhibit borers and grazers, such as in the Bahamas where stromatolites grow to a height of 2m.

Classification• Cyanobacteria are divided into three orders:• Order 1 Chroococcales: single cells or cells loosely bound into

gelatinous irregular colonies i.e. palmelloid colonies. Prochlorococcusmarinus is the dominant photosynthetic organism in tropical and temperate oceans.

• Order 2 Oscillatoriales: These are filamentous cyanobacteria without heterocysts.

• Order 3 Nostocales: filamentous cyanobacteria with heterocysts.

• http://www.dr-ralf-wagner.de/Blaualgen-englisch.html